Most people I know (pilots included) agree that oxygen is an essential requirement for life.
But many pilot’s understanding of how their body uses oxygen is less than adequate when considering its importance to safe flight. This understanding is not overly complicated and so it is simply a matter of reviewing the basics from time to time.
Pure oxygen is nonflammable, meaning that it will not burn (although it readily supports combustion of other flammable substances).
Oxygen combines with most other elements and is instrumental is the existence of hundreds of thousands of organic compounds. It’s little wonder then that our survival also depends on such an important element.
While the portion of the atmosphere we fly in is a fairly homogenous conglomeration of gases (what we call air), its’ primary components are nitrogen (~78%) and oxygen (~21%).. All other gases comprise the remaining one per cent, along with water vapor which varies from one per cent to about five per cent on a given day.
These percentages remain relatively constant with changes in altitude, but as we should all recall, the pressure exerted by air decreases as we climb. Since the weight of air at sea level on a standard day is 14.7 PSI and oxygen content is 21 per cent, it will exert a partial pressure of about 3.1 PSI.
This pressure is sufficient to oxygenate the blood at sea level. However, as we climb, this partial pressure decreases even though the percent content of oxygen remains the same. At some point the pressure is so low that oxygen can no longer be effectively forced into the bloodstream and we begin to experience symptoms of hypoxia.
As a result, in an unpressurized aircraft, we need to have supplemental oxygen at an altitude around 10,000 MSL and higher during prolonged periods of flight.
Fly high enough and not even supplemental oxygen is sufficient; we must now receive that oxygen under pressure to force it into the blood stream.
Since the symptoms of hypoxia (lack of oxygen to the organs) are sometimes subtle, it’s important to review what to expect with increases in altitude.
It is generally accepted that the first organs to be affected by the lack of oxygen are the eyes. The pilot may not notice these symptoms during daylight hours, and below about 10,000 feet it presents no significant problem for safety. On the other hand, at night above 5,000 to 6,000 feet our eyes are more susceptible to the effects of oxygen deprivation.
Above 10,000 feet prolonged flight generally leads to fatigue and sluggishness, so pilots flying unpressurized aircraft from 10,000 feet up to 13,000 feet for more than 30 minutes are required to have and use supplemental oxygen.
Bearing in mind that there is wide variability in susceptibility to hypoxia, above 15,000 feet the average pilot can add drowsiness, headaches, and poor judgment to the list of symptoms and also expect them to occur in as little as 30 minutes from arrival at those altitudes.
The first noticeable vision decrements starts around 18,000 feet with general blurriness. At 20,000 feet it only takes about 15 minutes for a loss of muscular control to set in. Judgment, reasoning and memory go right out the window as well.
Above around 22,000 feet it takes just minutes for the average person to lose consciousness. In an even worse case, there are about 15-20 seconds of useful consciousness at 40,000 feet. Keep in mind that above about 30,000 feet cabin altitude, the partial pressure of oxygen is too low for conventional supplemental oxygen systems, and so we must have a system that delivers oxygen under positive pressure to force the oxygen into the blood stream. One can think of a positive pressure system as effectively reducing the cabin altitude in which we are trying to breath.
Certain drugs, either in combination or alone, effectively increase the pilot’s susceptibility to hypoxia, so consult with your doctor prior to flying at altitude with any new medicines.
Individual symptoms to hypoxia vary from one person to another, so symptoms you hear about from a friend may or may not be the same as what you are likely to experience. Age also leads to changes in symptoms and the altitude of onset. The only good way to know exactly how you will respond to the onset of hypoxia is to experience it under controlled circumstances.
High altitude training, with an altitude chamber flight, is the best way to gain this knowledge about your own symptom set.
For most people, an additional symptom to those presented is cyanosis. Cyanosis is generally noticed in the extremities and lips. So a bluish appearance of lips and fingernails is usually a good sign of hypoxia onset.
Sometimes the first signs of hypoxia are a general inability to solve simple math problems.
Resolving hypoxia in a passenger or pilot is relatively easy if supplemental oxygen is available. Otherwise, a descent to lower altitude is warranted.
There are three basic types of oxygen systems for aircraft use: gaseous, liquid, and chemical. The basic light aircraft supplemental oxygen system is generally gaseous and uses the familiar bright green oxygen bottle (at least green at the top) to differentiate it from other less pure forms of the gas.
The usual medical oxygen is not nearly pure enough (too much water content) for use in aviation. Aviator’s oxygen will be in the neighborhood of 99.5 per cent pure to eliminate the possibility of water freezing within the delivery lines and potentially blocking the flow of oxygen to users.
The advantages of a gaseous system make it ideal for small aircraft. They are easy to handle, and transport. They are easy to service and relatively low cost to install. Some systems are portable, so you need not take it on every flight!
The only disadvantages over other oxygen systems are that connections are prone to leaks and a delivery pressure regulator is required due to the high pressure under which the oxygen is stored.
Oxygen is critical to safe flight and knowing one’s own response to a lack of oxygen is also very useful information. In lieu of knowing how you’ll respond to hypoxia, remember the common symptoms and watch for them when flying at high altitude. Realize that you cannot always recognize the onset of symptoms so follow the recommendations and regulations for oxygen use.